mingw/info/gdb/Registers.html

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  18
  19 <h3 class="section">Registers</h3>
  20
  21    <p>You can refer to machine register contents, in expressions, as variables
  22 with names starting with <code>$</code>.  The names of registers are different
  23 for each machine; use <code>info registers</code> to see the names used on
  24 your machine.
  25
  26      <dl>
  27 <dt><code>info registers</code>
  28      <dd>Print the names and values of all registers except floating-point
  29 registers (in the selected stack frame).
  30
  31      <br><dt><code>info all-registers</code>
  32      <dd>Print the names and values of all registers, including floating-point
  33 registers.
  34
  35      <br><dt><code>info registers </code><var>regname</var><code> ...</code>
  36      <dd>Print the <dfn>relativized</dfn> value of each specified register <var>regname</var>.
  37 As discussed in detail below, register values are normally relative to
  38 the selected stack frame.  <var>regname</var> may be any register name valid on
  39 the machine you are using, with or without the initial <code>$</code>.
  40 </dl>
  41
  42    GDB has four "standard" register names that are available (in
  43 expressions) on most machines--whenever they do not conflict with an
  44 architecture's canonical mnemonics for registers.  The register names
  45 <code>$pc</code> and <code>$sp</code> are used for the program counter register and
  46 the stack pointer.  <code>$fp</code> is used for a register that contains a
  47 pointer to the current stack frame, and <code>$ps</code> is used for a
  48 register that contains the processor status.  For example,
  49 you could print the program counter in hex with
  50
  51 <pre class="example">     p/x $pc
  52      </pre>
  53
  54 <p>or print the instruction to be executed next with
  55
  56 <pre class="example">     x/i $pc
  57      </pre>
  58
  59 <p>or add four to the stack pointer<a rel="footnote" href="#fn-1"><sup>1</sup></a> with
  60
  61 <pre class="example">     set $sp += 4
  62      </pre>
  63
  64    <p>Whenever possible, these four standard register names are available on
  65 your machine even though the machine has different canonical mnemonics,
  66 so long as there is no conflict.  The <code>info registers</code> command
  67 shows the canonical names.  For example, on the SPARC, <code>info
  68 registers</code> displays the processor status register as <code>$psr</code> but you
  69 can also refer to it as <code>$ps</code>; and on x86-based machines <code>$ps</code>
  70 is an alias for the <small>EFLAGS</small> register.
  71
  72    GDB always considers the contents of an ordinary register as an
  73 integer when the register is examined in this way.  Some machines have
  74 special registers which can hold nothing but floating point; these
  75 registers are considered to have floating point values.  There is no way
  76 to refer to the contents of an ordinary register as floating point value
  77 (although you can <em>print</em> it as a floating point value with
  78 <code>print/f $</code><var>regname</var><code></code>).
  79
  80    <p>Some registers have distinct "raw" and "virtual" data formats.  This
  81 means that the data format in which the register contents are saved by
  82 the operating system is not the same one that your program normally
  83 sees.  For example, the registers of the 68881 floating point
  84 coprocessor are always saved in "extended" (raw) format, but all C
  85 programs expect to work with "double" (virtual) format.  In such
  86 cases, GDB normally works with the virtual format only (the format
  87 that makes sense for your program), but the <code>info registers</code> command
  88 prints the data in both formats.
  89
  90    <p>Normally, register values are relative to the selected stack frame
  91 (see <a href="Selection.html#Selection">Selecting a frame</a>).  This means that you get the
  92 value that the register would contain if all stack frames farther in
  93 were exited and their saved registers restored.  In order to see the
  94 true contents of hardware registers, you must select the innermost
  95 frame (with <code>frame 0</code>).
  96
  97    <p>However, GDB must deduce where registers are saved, from the machine
  98 code generated by your compiler.  If some registers are not saved, or if
  99 GDB is unable to locate the saved registers, the selected stack
 100 frame makes no difference.
 101
 102    <div class="footnote">
 103 <hr>
 104 <h4>Footnotes</h4>
 105 <ol type="1">
 106 <li><a name="fn-1"></a>
 107 <p>This is a way of removing
 108 one word from the stack, on machines where stacks grow downward in
 109 memory (most machines, nowadays).  This assumes that the innermost
 110 stack frame is selected; setting <code>$sp</code> is not allowed when other
 111 stack frames are selected.  To pop entire frames off the stack,
 112 regardless of machine architecture, use <code>return</code>;
 113 see <a href="Returning.html#Returning">Returning from a function</a>.</p>
 114
 115    </ol><hr></div>
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